Tires are required to have not only a good steering stability on a dry road surface (dry grip performance) and a good steering stability on a wet road surface (wet grip performance), but also a low-temperature performance such as a steering stability under low-temperature conditions or on a snow-covered road surface (ice grip performance), i.e., it is required that the tires have a running stability with a high level under extensive environmental conditions.
In general, in order to enhance an ice grip performance of a rubber composition for the tires, it is effective to increase a contact area between the rubber composition and ice-snow. For this reason, it is required that the rubber composition exhibits an excellent flexibility in low-temperature conditions. In order to impart a good flexibility to the rubber composition, there is conventionally known the method of reducing an amount of carbon black compounded in the rubber composition or the method of adjusting an average particle size of the carbon black compounded in the range of from about 100 to about 200 nm. In these conventional methods, the rubber compositions can be improved in ice grip performance by imparting a flexibility thereto, i.e., by reducing an elastic modulus thereof in low-temperature conditions. However, on the other hand, these methods tend to suffer from such a problem that the rubber compositions are deteriorated in dry grip performance owing to hysteresis or reduction in elastic modulus in an ordinary temperature range.
On the other hand, in order to enhance the dry grip performance, there are known the method of using a rubber having a high glass transition temperature (Tg), e.g., a styrene-butadiene rubber in the rubber compositions, or the method of compounding a large amount of carbon black having an average particle size of from about 5 to about 100 nm in the rubber compositions. However, in these methods, there tends to occur such a problem that the rubber compositions are deteriorated in processability upon production owing to increase in viscosity thereof as well as in flexibility under low-temperature conditions, i.e., the rubber compositions are deteriorated in ice grip performance owing to increase in elastic modulus thereof.
Thus, the processability upon production and ice grip performance of the rubber compositions for tires have a contradictory relation with the dry grip performance thereof, and it is therefore considered that the rubber compositions are hardly improved in both of the properties in a well-balanced manner.
In Patent Document 1, as a rubber composition that can be improved in these properties in a well-balanced manner, there is described the rubber composition for tire treads which is compounded with a liquid polymer such as liquid polybutadiene.
Meanwhile, Patent Documents 2 and 3 describe the polymer obtained by polymerizing β-farnesene, but fail to have a sufficient study on practical applications thereof.